1. Field of the Invention
This invention relates generally to thermoplastic elastomer (TPE) materials. Thermoplastic elastomers are broadly defined as rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures. The invention more specifically relates to thermoplastic vulcanizates (TPV), which are thermoplastic elastomers with a cross-linked rubbery phase produced by the process of dynamic vulcanization. The thermoplastic vulcanizates of the invention are foamed materials produced by physical or chemical blowing agents, wherein processing characteristics and foam properties are improved by the inclusion of a modifier derived from polytetrafluoroethylene. The invention also relates to foamed articles obtainable by the process of the invention.
2. Description of the Prior Art
There has been considerable activity on the development of thermoplastic vulcanizate compositions, especially those based on polyolefin thermoplastic resins, which have good foaming properties, and on processes for producing foams having improved properties. U.S. Pat. No. 5,070,111, incorporated herein by reference, discloses a process of foaming thermoplastic elastomer compositions using water as the sole foaming agent. U.S. Pat. Nos. 5,607,629 and 5,788,889, both incorporated herein by reference, describe methods for the production of foamed thermoplastic elastomer profiles by extrusion with a water blowing agent. U.S. Pat. No. 5,824,400 discloses foamed thermoplastic elastomer compositions which incorporate styrenic elastomers. Published European Patent Application No. 0 860 465 teaches a method of foaming thermoplastic elastomers using a water containing chemical compound which releases water at temperatures above the melting point of the thermoplastic elastomer. Published European Patent Application 0 872 516 discloses the use of polypropylene resins having specific rheological properties to enhance the foaming performance of olefinic thermoplastic elastomers.
However, the problems of providing thermoplastic elastomer foams which are soft, with good surface smoothness, low water absorption, improved compression set and compression load deflection, and having fine and uniform cell structure have not been overcome by prior art.